A secure power supply includes the protection of the incoming power supply, as well as of the power distribution. These requirements must be met by a busbar distribution system, as well as by a cable system.
Therefore, power circuit breakers and an associated tripping system are adjusted in such a way that, when the need arises, only that power circuit breaker is tripped which is disposed directly upstream of the fault location for purposes of breaking fault currents. This selectivity should be limited to the section (the faulty electric circuit) that must be switched off in the event of a fault. All other system components should remain in operation. In this case, the tripping times of series-connected protection devices must be carefully adjusted to one another, and switching devices, as well as distribution systems (e.g., busbars or cables) must be able to conduct the short-circuit current during the total break time of the switching device, which is increased by the time delay required for the selectivity. This is referred to as the selective coordination of protection devices.
Protection and selectivity requirements, particularly in isolated power supply systems, are defined in classification and design regulations of certification firms, such as Germanischer Lloyd.
In an electrical system onboard a vessel, for example, the main source of electrical power is required to maintain the propulsion of the ship, and the main busbar can be divided into at least two sections which are normally connected by switches or other approved options. Whenever possible, the connections of the generators and of the coupled loads are to be uniformly apportioned among the busbar sections.
Groups of generators, loads and motor drives are connected to a supply line (for example, busbar). To electrically isolate at least two such groups in order to decouple them from one another in the case of a fault, an isolating point having a tie circuit breaker is devised. Subsequently to the opening of the tie circuit breaker (in the event of a fault), the two partial sections of a supply line are electrically independent of one another.
Rated currents on supply lines in an electrical system onboard a vessel can reach values of more than 8000 A, due to, for example, the high power demand at a voltage level of up to 690 V AC. This high power density pushes low-voltage switching devices to the limit of their performance capability. The tie circuit breaker, in particular, must be able to reliably switch off short-circuit currents of more than 100 kA. Conventional power circuit breakers are easily overloaded by such demands, so alternative solutions for limiting short-circuit currents are desirable.
Currents that occur during short circuits are extremely high. In one conventional solution, the current-carrying elements (e.g., cables or busbars) have correspondingly large conductor cross sections. Consequently, it is expensive to produce these circuit elements, particularly in view of the rising market prices for copper.
One alternative solution is to make use of a medium voltage in isolated low-voltage installations. However, due to safety considerations, medium voltage requires that the crew working on an isolated system include at least one authorized electrician.
German Patent Application Publication DE 10349552 A1 describes a short-circuit protection system used in low-voltage installations, which have a costly superconducting disconnector.
For high-speed short circuiting devices, German Patent Application Publication DE4438593 describes a short circuiting device having a thyristor-switching short circuit switch. German Patent Application Publication DE 4404074 describes a vacuum interrupter type switch. Some of these high-speed short circuiting devices are multiple short circuiting devices; others may be actuated only one single time (i.e., one-time short circuiting devices).
EP Patent Application Publication EP 1052 665 B1 and International Patent Application Publication WO 2000 62320 A1 describe a high-speed short circuiting device of the pyrotechnically operated type. This high-speed short circuiting device may induce a short circuit with an actuating time of less than 3 ms. The pyrotechnic drive drives a metallic pin through the stack of connection busbars, so that the phases are electrically and mechanically contacted among themselves within the actuating time, and a short circuit is produced. The pyrotechnically operated high-speed short circuiting devices are one-time short circuiting devices that must be replaced following a switching operation. In a low-voltage installation, an electrotechnically trained person may disassemble the actuated one-time short circuiting device and assemble a new high-speed short circuiting device.